1. Technical Field
The present invention relates to apparatus for enhancing the dynamic range of shockline-based sampling receivers.
2. Related Art
Two problems that arise in high frequency Vector Network Analyzers (VNA) that can affect the accuracy of measurements are intermodulation-product generation, created by the down-converters (which may include samplers), and inadequate channel to channel isolation that can limit the VNA's dynamic range. Use of shock-line based samplers in VNA receivers improves performance in both of these (among other) categories but these performance items are still an issue. One example of the use of shockline samplers in a VNA and how they may be further enhanced to increase isolation is described in U.S. Pat. No. 7,088,111 entitled “Enhanced Isolation Between Sampling Channels In A Vector Network Analyzer,” by K. Noujeim.
Shockline-based samplers, whether used in a VNA or other receivers to achieve very high frequency operation, have been the subject of other patents and numerous articles. For example, shockline devices for use in samplers are also described in the following: U.S. Pat. No. 6,894,581, entitled “Monolithic nonlinear transmission lines and circuits and sampling circuits with reduced shock-wave-to-surface-wave coupling,” by K. Noujeim; U.S. Pat. No. 5,014,018 entitled “Nonlinear transmission line for generation of picosecond electrical transients,” to Rodwell, et al.; and U.S. Pat. No. 7,170,362 entitled “Ultrafast sampler with non-parallel shockline,” to Agoston, et al.
FIG. 1 shows an RF block of a prior art VNA using shockline-based samplers. Signal generator 110 provides a source signal to power splitter 112 through a source resistance 111. The power splitter 112 splits the source signal into separate signals for driving different test channel and reference channel samplers. In the example shown, the source signal is split into four signals to drive two test channel samplers and two reference channel samplers. The shockline-based samplers use nonlinear transmission lines (NLTLs) 151-154 which can receive a continuous wave (CW) local oscillator (LO) signal from the signal generator through the power splitter. Each NLTL compresses the falling edge of the LO signal creating a series of sharp step-function-like wavefronts, or shocks. The pulse forming network of the sampler can then be used to differentiate these shocks, resulting in electrical pulses that are used as to gate the sampler.
In operation, a VNA sources a sweeping RF signal that can be applied to a device under test (DUT) 180 at ports 182, 184, or both. The DUT causes a transmission signal or reflection signal that is received by one of samplers 161-164 of the VNA. Each sampler is gated by the LO pulses generated by the NLTLs 151-154. An intermediate frequency (IF) signal comprising a series of sampled data is then created and transmitted from each sampler as IF1, IF2, IF3 and IF4 is illustrated in FIG. 1.
As shown in FIG. 1, a NLTL is made up of a high impedance transmission lines loaded periodically with varactor diodes forming a propagation medium whose phase velocity, and therefore time delay, is a function of the instantaneous voltage. Shockline-based samplers are attractive because of features such as ultra-wideband RF bandwidth and frequency scalability when compared with typical samplers that use step-recovery diode (SRD) circuits instead of the shocklines.
A pulse forming network differentiates shocks generated by the shocklines to create electrical pulses forming LO signals that gate a Schottky-based sampler. But the sampler is well known for generating spurious products that result when an RF signal undergoes partial reflection at the sampler's RF port. In a VNA context, these spurious products emerge from the sampler's RF port and make their way to the device under test (DUT) to create measurement errors. The rich harmonic content of a narrow sampling pulse mixes in the diodes of the sampler with these RF signals to generate spurious products. These products proceed through paths in the measurement system and can re-convert to the system IF in another VNA port's receiver. This could lead to an unrealistic measurement due to the conversion of these spurious products creating an IF similar to that produced by the intended signals.
FIG. 2 shows an example of spurious signal generation. As shown in FIG. 2, a LO source signal 201 is provided to a NLTL shockline 202 that provides a pulse signal to pulse forming network 203. A pulse-forming network differentiates the pulse signal to gate a sampler 204, in this example Sampler 2. An RF signal, shown as fRF, can be either provided through DUT 205 from another port or reflected from the DUT 205 and received at the RF port of Sampler 2. The reflected RF signal can mix with LO products in Sampler 2, generating a spurious signal, fspur, shown as comprising |mfRF±nfLO±pfIF| where m, n, and p are integers. This spurious signal may then be passed back through the passband of the DUT and be received by a sampler measuring the transmission signal from the DUT, such as Sampler 3. In addition, the spurious signal could move through the LO distribution chain of Sampler 2 manifesting itself as defects in RF-LO isolation, such as along path 186 shown in FIG. 1.
Previous techniques for improving isolation between channels include the use of isolating devices in the sampler's RF path. This could, however, be very expensive in a broadband system and could worsen the overall dynamic range of the system by introducing additional non-idealities, such as noise or compression. In addition, the use of the RF amplifiers could require additional RF-LO isolation in the LO chain itself. In the configuration of FIG. 1, a combination of band-pass filters, reverse isolation elements, and amplifiers are used in one or more stages to increase isolation between channels by limiting RF leakage between channels. Band pass filters 141-144 may be configured to reduce all signals outside a particular frequency band (fL-fH), preventing or reducing RF signals that fall outside of the frequency band from leaking from the RF port of one sampler to another. Isolators 121-124 or amplifiers 131-134 may also be added and configured to allow signals falling inside the frequency band (fL-fH) to travel along only a single direction from power splitter 112 through the band-pass filter and into the non-linear transmission line. As a result, RF signals falling inside or outside the frequency band and traveling through the power splitter from the RF port of one sampler to the RF port of another sampler will be blocked or greatly reduced. Additionally, amplifiers 186-192 may be added in the RF path to prevent or reduce spurious products from being applied to a DUT and being passed through its passband to the IF of another sampler. Multiple stages of band pass filters, isolators, and amplifiers may be used to reduce signal leakage between channels and achieve similar performance gains.
While adding additional components as shown in FIG. 1 may improve isolation, such systems to improve isolation have drawbacks. Multiple stages of filters, isolators, and amplifiers added to achieve sufficient isolation can significantly increase cost and power consumption, particularly in broadband systems. The addition of isolation components and multiple amplifier stages can further worsen the overall dynamic range of the receiver.
Another technique to increase channel isolation includes the sequential turn-off of the samplers. The sequential turn off allows samplers not being used for that particular measurement to be disabled so they are not generating spurious products. This solution may require additional sampler power supply ports to accomplish the turn off and may reduce available intermediate frequency (IF) bandwidth due to loading.
In another similar technique to increase isolation, the LO power could be turned on and off to remove the LO from channels not in use. This would again prevent spurious signals from being generated in paths not being used. But this technique can be complicated since high power levels at high frequencies are often involved in the LO which cannot be readily switched on and off. There may also be the thermal-transient and power stability issues in trying to turn rapidly off and on LO power as would happen in a high-speed measurement system.